Electrolysis process can generate hydrogen, oxygen, ozone, hydrogen peroxide and the like in aqueous solution systems by controlling chemical reaction on an electrode surface utilizing clean electric energy, and is a basic technology widely used in electrolysis of sodium chloride, electroplating, extraction of metals, and the like as industrial electrolysis. Recently, the electrolysis process is being utilized for waste water treatment because it is possible to indirectly decompose organic pollutants, or to adsorb the pollutants onto an electrode, thereby directly electrolyzing them.
On the other hand, it is known that according to anodic oxidation reaction in electrolysis, oxidizers (for example, effective chlorine and ozone) effective for water treatment are formed and that active species such as an OH radical are partially generated. Water containing them is generally used as a name such as active water, functional water, ionic water, and sterile water (Basic Knowledge of Strongly Acidic Electrolyzed Water, Ohm Co.).
The electrolysis process is widely put to practical use. However, it is pointed out that objective reaction does not sufficiently proceed depending upon electrode materials. In general, according to anodic oxidation reaction of electrolysis in an aqueous solution, the electrolysis proceeds with water as a raw material, to obtain an electrolytic product. However, in many cases, in electrode catalysts having high reactivity against discharge of water, oxidation of other co-existing substances does not readily proceed.
As materials of an electrode (anode) for electrolysis to be used for carrying out oxidation, lead oxide, tin oxide, platinum, DSA, carbon, and the like are used. Materials that can be used as an electrode substrate are required to have a long life and have corrosion resistance such that staining on the treated surface does not occur. Materials for an anode collector are limited to valve metals such as titanium and alloys thereof Electrode catalysts are also limited to noble metals such as platinum or iridium, and oxides thereof Even if such an expensive material is used, it is known that when an electric current is applied, the material is exhausted corresponding the current density or current-carrying time and eluted into an electrolytic solution. Thus, electrodes having more excellent corrosion resistance are being desired.
Graphite and amorphous carbon materials have hitherto been used as an electrode material. However, these materials are markedly exhausted, in particular, under anodic polarization. Diamond is excellent in thermal conductivity, optical transmission, and durability against high temperatures and oxidation. In particular, since it is possible to control electric conductivity by doping, diamond is promising as a semiconductor device or energy conversion element.
Recently, it is reported that diamond has stability as an electrochemical electrode in an acidic electrolytic solution, and it is suggested that diamond is far excellent as compared with other carbon materials (see Swain et al., Journal of Electrochemical Society, Vol. 141, 3382-(1994)). Basic electrochemical properties of diamond are described in detail in Electrochemistry and Industrial Physical Chemistry, p389, Vol. 67, No.4 (1999).
U.S. Pat. No. 5,399,247 suggests that organic waste water can be decomposed using diamond as an anode material. JP-A-2000-226682 proposes a method of carrying out water treatment using conductive diamond as an anode and a cathode. Further, JP-A-2000-254650 proposes a method of carrying out water treatment using conductive diamond as an anode and a gas diffusion cathode for generating hydrogen peroxide as a cathode.
Any industrial application of a diamond electrode in high potential region under high current density has not yet been reported. However, recently, it is reported that the diamond electrode is inert against decomposition reaction of water and forms ozone in addition to oxygen (see JP-A-11-269685).
From these researches, according to the electrolysis process using diamond as an electrode, an enhancement of the efficiency is expected as compared with the case using a conventional electrode. On the other hand, improvements have been desired from the following viewpoint of practical use.
As a method of preparing diamond films, a hot filament CVD method, a microwave plasma CVD method, a plasma arc jet method, a PVD method, and the like are developed. In the CVD method as a general production method of diamond, since a high-temperature reduction step of 700° C. or higher is employed, it is required that a coefficient of thermal expansion of a substrate is closed to that of diamond. For substrates of a diamond electrode, metallic silicon whose coefficient of thermal expansion is closed to that of diamond is usually used. This metallic silicon is low in mechanical strength such as brittleness, and its size is limited, so that it is difficult to make it large in size. Since the shape of electrodes to be used for industrial electrolysis is complicated, it is also preferred to use metallic substrates that are easy for processing and high in mechanical strength. In particular, as metals that are stable in acidic solutions in an anodic potential region, valve metals such as titanium, zirconium or niobium, or its alloy can be used.
Hydrogen atmosphere at high temperature region of 700° C. or higher is formed in a reaction vessel, and there is the possibility that general metal materials undergo hydrogen brittleness. It is investigated to use niobium or tantalum substrate, considering that in the use thereof, hydrides reversely isolate under such conditions, and absorption amount of hydrogen decreases, thereby risk of brittleness is avoided. Anti-corrosion of niobium is slightly inferior to that of tantalum, but is excellent in specific strength and costs. It is necessary for titanium and zirconium to add steps of maintaining in vacuum at high temperature region and dehydrogenating, in order to avoid hydrogen brittleness.
There is a scratching method as a pre-treatment generally conducted in order to securely form a diamond film on a substrate. Different from general vacuum deposition, diamond nuclei do not substantially generate on a mirror-polished substrate. For generation of nuclei, sites having high energy, such as transformation or step, to be starting point are required. Therefore, scratching treatment is generally conducted using diamond or SiC particles, having high hardness. At the initial stage, a manual polishing by diamond paste having a particle diameter of submicron meter to several tens μm was conducted as the scratching treatment. At present, the scratching method is improved, and ultrasonic treatment is established, in which an ultrasonic washer is used, about 1 g of particles is introduced into about 20 cc of alcohol, and the washer is lightly beaten such that particles vibrate while rolling, thereby forming scratches (S. Yugo, New Diamiond, Vol. 7, No. 1, 7 (1991)). The maximum nucleus generation density obtained by this technique is about 109/cm2 by microwave plasma CVD method.
From the analysis of this ultrasonic scratching method, a bias method (electric field treatment) has been developed by Yugo et al (S. Yugo, T. Tanai, T. Kimura and T. Muto: Appl. Phys. Lett., Vol. 58, No. 11, p1038 (1991)). This method is as follows. In plasma CVD method, by applying a direct current voltage of about −100V, surface washing is conducted by hydrogen ions or the like, defects to be nucleus generation site, such as transformation or step, are introduced, amorphous carbon other than diamond is removed, migration of carbon ions is activated, and formation of carbon cluster is accelerated. This enables the nucleus generation density to be about 1011/cm2.
On the other hand, a seeding method was invented. This method appears to be derived from the phenomena that diamond particles themselves or diamond particles remained in the scratching treatment form nuclei, and diamond film easily grows. This method conducts formation of CVD diamond film after previously applying nanodiamond particles having a particle diameter of 1-50 nm that are easy to apply as a sol (dispersed colloidal solution) to a substrate. At the initial stage, in the seeding method, investigation of coating method proceeded from dip method or spinner method. Recently, by using nanodiamond particles having an average particle diameter of 5-10 nm, and ultrasonic vibration treatment having a recoagulation suppression effect of fine particles in dispersed colloidal solution and an embedding effect of fine particles into a substrate, the nucleus generation density having high density 100 times (1011/cm2) or more of that in the scratching treatment has been obtained (H. Kurokawa, et al: Electric Society, Metal and Ceramics Research Meeting Materials, Vol. MC-97-3, 12-18 (1997)).
From the standpoint of the result, the scratching treatment and seeding treatment, using both diamond particles and ultrasonic vibration are very similar treatment, and are frequently confused. The great difference is improvement of nucleus generation density proceeded with diamond particle diameter in time series.
As a pre-treatment of a substrate to be subjected to the seeding treatment, in many cases a metallic silicon substrate is generally subjected to only acid washing for the purpose of removing oxide film or oxide layer formed in the atmosphere. Valve metals suitable for use in electrolysis are subjected to sandblast treatment prior to acid washing. The reasons for this are that unevenness of several μm to several tens μm is formed on the surface a substrate to absorb difference in coefficient of thermal expansion between the substrate and diamond when forming CVD diamond film, thereby preventing peeling of the film from the substrate, and actual electrolysis current density decreases.
In order to provide a stable anode, it is performed to maintain durability of a substrate. For the purposes of obtaining adhesion of a diamond film to a substrate and protecting the substrate, there is the case that it is preferable to form various interlayers on the substrate surface (see JP-A-9-268395). It is disclosed that the effect of the interlayer is a basic technique for prolonging the life of a noble metal oxide electrode in an acidic electrolytic bath (see JP-B-60-21232 and JP-B-60-22074). However, even if such an oxide interlayer is formed, since radicals such as hydrogen generate under CVD diamond synthesis conditions, the greater part of the interlayer is reduced, and therefore, it is not simple to apply the subject technique.
If the interlayer is a carbide originated from the substrate, it is expected to increase adhesion of diamond film from the relationships between substrate and carbide grown from the substrate, and between carbide and diamond generated from the carbide as a nucleus. However, actually in many cases the carbide is inferior in corrosion resistance to an oxide when voltage is applied to an anode in a strongly acidic solution (For example, in the case of NbC, see  ,  ,  : Zashch Met, 458-461 (198705-198706)). Carbide is liable to be formed on a substrate contacting hydrocarbon radicals or diamond in high temperature atmosphere. Therefore, it is necessary to pay attention when using as an anode.
However, depending on the applied field, even such an improved diamond electrode has poor life, and cannot answer the problems. As a result of investigation of this cause, it has been confirmed that a large-sized electrode involves difference in coefficient of thermal expansion and scattering in quality of diamond due to heterogeneity of CVD device (precipitation of non-diamond component), and defects such as pinholes or cracks unavoidably occur.
To obtain a diamond film having good film-forming property, i.e., a uniform and defect-free diamond film, it is an important condition that generation of high density nucleus can efficiently be conducted with good reproducibility by controlling nucleus generation.